semtimedop • man page

semop, semtimedop - System V semaphore operations

semtimedop • man page

semop, semtimedop - System V semaphore operations

semtimedop (2)

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Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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(The comments found at the beginning of the groff file "man2/semtimedop.2".)

semop()
performs operations on selected semaphores in the set indicated by
semid.
Each of the
nsops
elements in the array pointed to by
sops
is a structure that
specifies an operation to be performed on a single semaphore.
The elements of this structure are of type
struct sembuf,
containing the following members:

Flags recognized in
sem_flg
are
IPC_NOWAIT
and
SEM_UNDO.
If an operation specifies
SEM_UNDO,
it will be automatically undone when the process terminates.

The set of operations contained in
sops
is performed in
array order,
and
atomically,
that is, the operations are performed either as a complete unit,
or not at all.
The behavior of the system call if not all operations can be
performed immediately depends on the presence of the
IPC_NOWAIT
flag in the individual
sem_flg
fields, as noted below.

Each operation is performed on the
sem_num-th
semaphore of the semaphore set, where the first semaphore of the set
is numbered 0.
There are three types of operation, distinguished by the value of
sem_op.

If
sem_op
is a positive integer, the operation adds this value to
the semaphore value
(semval).
Furthermore, if
SEM_UNDO
is specified for this operation, the system subtracts the value
sem_op
from the semaphore adjustment
(semadj)
value for this semaphore.
This operation can always proceed---it never forces a thread to wait.
The calling process must have alter permission on the semaphore set.

If
sem_op
is zero, the process must have read permission on the semaphore
set.
This is a "wait-for-zero" operation: if
semval
is zero, the operation can immediately proceed.
Otherwise, if
IPC_NOWAIT
is specified in
sem_flg,
semop()
fails with
errno
set to
EAGAIN
(and none of the operations in
sops
is performed).
Otherwise,
semzcnt
(the count of threads waiting until this semaphore's value becomes zero)
is incremented by one and the thread sleeps until
one of the following occurs:

*

semval
becomes 0, at which time the value of
semzcnt
is decremented.

*

The semaphore set
is removed:
semop()
fails, with
errno
set to
EIDRM.

*

The calling thread catches a signal:
the value of
semzcnt
is decremented and
semop()
fails, with
errno
set to
EINTR.

If
sem_op
is less than zero, the process must have alter permission on the
semaphore set.
If
semval
is greater than or equal to the absolute value of
sem_op,
the operation can proceed immediately:
the absolute value of
sem_op
is subtracted from
semval,
and, if
SEM_UNDO
is specified for this operation, the system adds the absolute value of
sem_op
to the semaphore adjustment
(semadj)
value for this semaphore.
If the absolute value of
sem_op
is greater than
semval,
and
IPC_NOWAIT
is specified in
sem_flg,
semop()
fails, with
errno
set to
EAGAIN
(and none of the operations in
sops
is performed).
Otherwise,
semncnt
(the counter of threads waiting for this semaphore's value to increase)
is incremented by one and the thread sleeps until
one of the following occurs:

*

semval
becomes greater than or equal to the absolute value of
sem_op:
the operation now proceeds, as described above.

*

The semaphore set is removed from the system:
semop()
fails, with
errno
set to
EIDRM.

*

The calling thread catches a signal:
the value of
semncnt
is decremented and
semop()
fails, with
errno
set to
EINTR.

On successful completion, the
sempid
value for each semaphore specified in the array pointed to by
sops
is set to the caller's process ID.
In addition, the
sem_otime
is set to the current time.

semtimedop()

semtimedop()
behaves identically to
semop()
except that in those cases where the calling thread would sleep,
the duration of that sleep is limited by the amount of elapsed
time specified by the
timespec
structure whose address is passed in the
timeout
argument.
(This sleep interval will be rounded up to the system clock granularity,
and kernel scheduling delays mean that the interval
may overrun by a small amount.)
If the specified time limit has been reached,
semtimedop()
fails with
errno
set to
EAGAIN
(and none of the operations in
sops
is performed).
If the
timeout
argument is NULL,
then
semtimedop()
behaves exactly like
semop().

Note that if
semtimedop()
is interrupted by a signal, causing the call to fail with the error
EINTR,
the contents of
timeout
are left unchanged.

RETURN VALUE

If successful,
semop()
and
semtimedop()
return 0;
otherwise they return -1
with
errno
indicating the error.

ERRORS

On failure,
errno
is set to one of the following:

E2BIG

The argument
nsops
is greater than
SEMOPM,
the maximum number of operations allowed per system
call.

EACCES

The calling process does not have the permissions required
to perform the specified semaphore operations,
and does not have the
CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.

EAGAIN

An operation could not proceed immediately and either
IPC_NOWAIT
was specified in
sem_flg
or the time limit specified in
timeout
expired.

EFAULT

An address specified in either the
sops
or the
timeout
argument isn't accessible.

EFBIG

For some operation the value of
sem_num
is less than 0 or greater than or equal to the number
of semaphores in the set.

EIDRM

The semaphore set was removed.

EINTR

While blocked in this system call, the thread caught a signal; see
signal(7).

EINVAL

The semaphore set doesn't exist, or
semid
is less than zero, or
nsops
has a nonpositive value.

ENOMEM

The
sem_flg
of some operation specified
SEM_UNDO
and the system does not have enough memory to allocate the undo
structure.

ERANGE

For some operation
sem_op+semval
is greater than
SEMVMX,
the implementation dependent maximum value for
semval.

VERSIONS

semtimedop()
first appeared in Linux 2.5.52,
and was subsequently backported into kernel 2.4.22.
Glibc support for
semtimedop()
first appeared in version 2.3.3.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

NOTES

The inclusion of
<sys/types.h>
and
<sys/ipc.h>
isn't required on Linux or by any version of POSIX.
However,
some old implementations required the inclusion of these header files,
and the SVID also documented their inclusion.
Applications intended to be portable to such old systems may need
to include these header files.

The
sem_undo
structures of a process aren't inherited by the child produced by
fork(2),
but they are inherited across an
execve(2)
system call.

semop()
is never automatically restarted after being interrupted by a signal handler,
regardless of the setting of the
SA_RESTART
flag when establishing a signal handler.

A semaphore adjustment
(semadj)
value is a per-process, per-semaphore integer that is the negated sum
of all operations performed on a semaphore specifying the
SEM_UNDO
flag.
Each process has a list of
semadj
values---one value for each semaphore on which it has operated using
SEM_UNDO.
When a process terminates, each of its per-semaphore
semadj
values is added to the corresponding semaphore,
thus undoing the effect of that process's operations on the semaphore
(but see BUGS below).
When a semaphore's value is directly set using the
SETVAL
or
SETALL
request to
semctl(2),
the corresponding
semadj
values in all processes are cleared.
The
clone(2)CLONE_SYSVSEM
flag allows more than one process to share a
semadj
list; see
clone(2)
for details.

The semval, sempid, semzcnt, and semnct values
for a semaphore can all be retrieved using appropriate
semctl(2)
calls.

Semaphore limits

The following limits on semaphore set resources affect the
semop()
call:

SEMOPM

Maximum number of operations allowed for one
semop()
call.
Before Linux 3.19,
the default value for this limit was 32.
Since Linux 3.19, the default value is 500.
On Linux, this limit can be read and modified via the third field of
/proc/sys/kernel/sem.
Note:
this limit should not be raised above 1000,
because of the risk of that
semop()
fails due to kernel memory fragmentation when allocating memory to copy the
sops
array.

SEMVMX

Maximum allowable value for
semval:
implementation dependent (32767).

The implementation has no intrinsic limits for
the adjust on exit maximum value
(SEMAEM),
the system wide maximum number of undo structures
(SEMMNU)
and the per-process maximum number of undo entries system parameters.

BUGS

When a process terminates, its set of associated
semadj
structures is used to undo the effect of all of the
semaphore operations it performed with the
SEM_UNDO
flag.
This raises a difficulty: if one (or more) of these semaphore adjustments
would result in an attempt to decrease a semaphore's value below zero,
what should an implementation do?
One possible approach would be to block until all the semaphore
adjustments could be performed.
This is however undesirable since it could force process termination to
block for arbitrarily long periods.
Another possibility is that such semaphore adjustments could be ignored
altogether (somewhat analogously to failing when
IPC_NOWAIT
is specified for a semaphore operation).
Linux adopts a third approach: decreasing the semaphore value
as far as possible (i.e., to zero) and allowing process
termination to proceed immediately.

In kernels 2.6.x, x <= 10, there is a bug that in some circumstances
prevents a thread that is waiting for a semaphore value to become
zero from being woken up when the value does actually become zero.
This bug is fixed in kernel 2.6.11.

EXAMPLE

The following code segment uses
semop()
to atomically wait for the value of semaphore 0 to become zero,
and then increment the semaphore value by one.

SEE ALSO

COLOPHON

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